Immersion heater devices of the above-described type are mainly used, though not exclusively, for regulating the temperature of water inside aquariums so as to reproduce the conditions of the natural habitat of fishes placed therein.
These devices, which are generally supplied with electrical energy, must comply with strict safety provisions since they are potentially handled by the user during the normal maintenance of the aquarium. In particular, the standards in force in some countries require that the elements of the devices be subjected to various strength and resistance tests, such as mechanical strength (for example, resistance to collisions or impacts) and heat resistance (for example, resistance to thermal shock), therefore imposing very precise structural features.
Most of the known devices comprise a glass casing in contact with the water, containing the electrical resistor which can heat the water by radiation. However, glass presents some disadvantages, such as the possibility of being easily broken as a result both of accidental collisions against elements placed inside the aquarium (such as stones, rocks, etc.) and thermal shocks produced, for example, by the immersion in water of a device, a device which has accidentally been left switched on out of the water, or by filling the aquarium with cold water. In order to prevent breakages due to thermal shock, safety thermostats are generally provided in the glass devices; these interrupt the energy supply of the resistor when a particular predetermined temperature value is reached at the surface of the casing. However, the provision of such a thermostat, in order to ensure correct operation of the device when it is immersed in water, involves an increase in the longitudinal dimensions of the casing, making the device costly and bulky.
There are also known heating devices which have a metal casing, but which need to be grounded to earth in order to comply with the safety standards, and which further involve a high cost in comparison with the glass devices due to the material used.
An example of this prior art is disclosed in FR-A-2401585.
Finally, a device having a casing of plastic material is known from Italian Patent No. 1300229. This device has never been marketed, however, since it is not suitable for withstanding high temperatures, such as, for example, those reached by the device should it remain live out of the water, at which temperatures the plastic material softens. It is possible to overcome this disadvantage by producing the casing from heat-resistant plastic materials, such as thermo-resistant resins, which have the disadvantage, however, of high cost.